Great question. The GoPro 9 is an additional $100 above the 8, so that's $170 above the 7. Is the GoPro 9 $170 USD better than the 7? I don't know, but would love to know thoughts.
I would bite the bullet and get the 9. The 5 6 7 all share the same case design. The 8 has a unique case that isn’t the compatible the the ones before or after. Hopefully the new 9 will stabilize the case design for a few years.